Friction stir processing tool with different shoulder regions

ABSTRACT

A friction stir processing tool having a shoulder, which is implemented on a body, and which has a friction surface for pressing against at least one workpiece at least partially provided with a surface coating, and a friction reducing unit for reducing the friction resistance between the surface coating and the friction surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to European PatentApplication No. 13 002 961.4, filed Jun. 10, 2013, the entire disclosureof which is herein expressly incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

Exemplary embodiments of the invention relate to a friction stirprocessing tool having a shoulder implemented on a body, which has afriction surface for pressing against at least one workpiece, which isat least partially provided with a surface coating. Furthermore, theinvention relates to a friction stir processing device provided withsuch a friction stir processing tool. Finally, the invention relates toa friction stir processing method, in particular a friction stir weldingmethod, which can be carried out therewith.

The invention is in the field of friction stir processing and inparticular friction stir welding. Friction stir welding (FSW) isincreasingly used in aerospace technology, in rail traffic technology,entertainment electronics, and in automotive engineering. It is asimple, clean, and innovative joining method, which is distinguished bya high potential for automation and a high efficiency, whereby theproduction costs are decreased and the weight of structures producedtherefrom is reduced.

Friction stir welding was first described in PCT document WO 93/19935.In this case, two workpieces to be welded to one another are broughtinto contact and held in this position. A welding pin or a pin-shapedprojection of a corresponding tool is inserted into the connectingregion of the workpieces with rotating movement until a shoulderarranged above the welding pin on the tool rests on the surface of theworkpieces. In this case, friction heat is generated by the relativemovement between tool and workpieces, so that adjacent material regionsin the connecting region assume a plasticized state. The tool is movedforward along a connecting line of the workpieces while the rotatingwelding pin is in contact with the connecting region, so that thematerial located around the welding pin plasticizes and subsequentlyconsolidates. Before the material hardens completely the welding pin isremoved from the connecting region or the workpieces, respectively.

Materials, for example, metals, their alloys, metal materialcomposites—so-called MMCs—or suitable plastic materials can be welded inthis manner as a butt joint, lap joint, or T-joint connection. Spotconnections can also be generated, wherein a forward movement of therotation with the welding pin in contact with the connecting region or atranslational relative movement between rotating welding pin andworkpieces does not occur.

However, the technique of friction stir processing is also applied inthe repair, processing, and finishing of workpieces. In this case, asdescribed above, a pin-shaped projection is inserted into at least oneworkpiece with rotating movement (i.e., welding is performed in solidmaterial), to modify the workpiece at least in the contact region of thewelding pin. For repair purposes, the rotating welding pin is insertedinto a crack of a workpiece, for example.

German patent document DE 10 2005 030 800 B4 discloses a friction stirprocessing tool for friction stir welding for the purpose of connectinga first workpiece and a second workpiece along a weld seam as aconnecting line and a corresponding friction stir welding method. Inthis known method, the pin and the shoulder or a body of the toolbearing the shoulder are rotatable in relation to one another. Inparticular, the shoulder is fixed during the welding, so that it doesnot execute a rotational movement (rotational speed n=0). Less frictionheat is thus generated in the region of the surface.

Many novel possible welding tasks, for example, the connection ofworkpieces of entirely differing material consistency, result by way ofthis welding method known from German patent document DE 10 2005 030 800B4 and the corresponding tool. For example, metals having low meltingtemperatures can be welded to metals having higher welding temperatures.This is possible in that the friction heat can be monitored better andtherefore the material that softens early can be kept in a plasticizedand non-liquid state. Thus, for example, connections are even possiblebetween aluminum workpieces and titanium workpieces. Mixed connectionsof aluminum-steel are also possible.

However, if workpieces having a surface coating are processed in thefriction stir processing method, this leads, as a result of the relativemovement between shoulder and workpiece, to the surface coating beingdamaged.

For example, using the friction stir processing tool described in Germanpatent document DE 10 2005 030 800 B4 and the friction stir weldingmethod described therein, welding can be performed between a galvanizedsteel plate and an aluminum plate. The friction of the shoulder duringthe advance of the friction stir processing tool over the connectingline damages the zinc layer of the steel plate in many cases.

Damage to the coating by the shoulder of the friction stir processingtool, for example, the welding tool, generally occurs if friction stirwelding processes or friction stir processing processes in general areapplied to coated joined parts or workpieces.

Heretofore, the coating of the joined parts or the workpiece was locallyreapplied in the weld region after completed welding.

Exemplary embodiments of the invention are directed to friction stirprocessing methods on coated workpieces having simpler processes.

According to a first aspect, the invention provides a friction stirprocessing tool having a shoulder implemented on a body, which has afriction surface for pressing against at least one workpiece, which isat least partially provided with a surface coating, and a frictionreducing unit for reducing the friction resistance between the surfacecoating and the friction surface.

By the reduction of the friction resistance on the friction surfaceregion, damage to coatings of a workpiece to be processed may bedecreased or avoided, so that a subsequent further local application ofthe coating can be avoided.

According to one preferred embodiment of the invention, a region of theshoulder that comes into contact with the surface coating of thematerial to be processed is implemented as more yielding than thesurface coating.

It is preferable for the friction reducing unit to have at least onesupport element for supporting at least a partial region of the frictionsurface such that at least the partial region of the friction surface ismore yielding than the surface coating of the friction surface.

The friction reducing unit is preferably implemented such that thefriction forces or the friction load on one partial region of thefriction surface of the shoulder are selectively kept lower than onanother partial region of the friction surface of the shoulder.

According to a further aspect, the invention provides a friction stirwelding tool having a multipart tool shoulder.

This is advantageous for friction stir welding of mixed connections inparticular, in which one workpiece is provided with a sensitive surfacecoating and another workpiece is not.

For example, a first workpiece is formed from galvanized steel plate,which is to be welded onto a second workpiece made of aluminum.

There are many further possible examples of pairs of first and secondworkpieces, of which a first workpiece is provided with a surfacecoating.

It is preferable for the friction stir processing tool to be providedwith a shoulder implemented on a body, which has a friction surface forpressing against a first workpiece and a second workpiece to beconnected to one another.

It is preferable for the friction surface to comprise a first surfaceregion and a second surface region, and the friction reducing unit to beimplemented only on a partial region of the body such that the frictionresistance on this first surface region of the friction surface is lessthan the friction resistance on the second surface region of thefriction surface.

The first workpiece provided with the surface coating can be engagedwith the first surface region, while the vertical process forces aretransferable more via the second surface region.

The reduction of the friction can be performed in various ways.Provision with a friction-reducing layer is conceivable. However, thevertical contact pressure forces on the first surface region arepreferably less than on the other surface region. This may be achievedby greater yielding of the first surface region than the second surfaceregion.

It is preferable for the support element to be implemented to supportthe first surface region and to be softer or more yielding than asupporting surface of the second surface region.

It is preferable for the first surface region to be implemented on aregion of the body formed from a plastic material and the second surfaceregion to be formed on a region of the body formed from a metal and/orceramic material.

According to one embodiment, it is preferable for the first surfaceregion to be formed by a first half of the friction surface and thesecond surface region to be formed by the second half of the frictionsurface.

Such an embodiment is particularly advantageous for friction stirwelding methods, in which a movement of the friction stir processingtool occurs centrally to the connecting line between the workpieces.

According to a further embodiment, it is preferable for one of thesurface regions, preferably the first surface region, to be implementedon a partial segment of the friction surface which occupies less thanhalf of the friction surface.

It is preferable for a partition line, which separates the first surfaceregion from the second surface region, to extend off-center to thefriction surface. The partition line is preferably arranged such thatthe second surface region extends by a predefined distance beyond acenter line, which can be arranged in an advance direction for thefriction stir processing tool, of the friction surface.

Embodiments having an off-center partition between surface regions areadvantageous, for example, for friction stir welding methods in whichthe shoulder is applied and moved off-center to a connecting linebetween workpieces. This is advantageous, for example, during thewelding of workpieces made of materials having significantly differingmelting points. The material having the lower melting point plasticizesat lower temperatures than the material having the higher melting point.An off-center application and movement depending on the materialcomposition can therefore be advisable; it is accordingly advantageousto adapt the distribution of the surface regions.

It is preferable for a pin, which protrudes from the shoulder and isdrivable to rotate, to be provided, which is rotatable in relation tothe shoulder.

It is preferable for the shoulder to be implemented, during a frictionstir welding method for connecting a first workpiece and a secondworkpiece along a weld seam, to be pressed at a rotational speed n=0against the surface of the first and/or the second workpiece.

According to a further aspect, the invention provides a friction stirprocessing device comprising a friction stir processing tool accordingto one of the above-explained embodiments and a tool guiding unit forguiding and moving the friction stir processing tool in an advancedirection.

The friction stir processing device is particularly preferably afriction stir welding device, wherein the tool guiding unit isimplemented for the purpose of guiding the friction stir processing toolalong a connecting line between workpieces to be connected to oneanother to form a weld seam.

According to a further aspect, the invention provides a friction stirprocessing method for carrying out a friction stir process on at leastone workpiece provided with a surface coating, comprising: using afriction stir processing tool according to one of the above-explainedembodiments, wherein the shoulder is selected and/or implementeddepending on the surface coating such that, when it is pressed againstthe surface coating and moved on the surface coating in an advancedirection during the friction stir process, it does not damage or atleast does not completely abrade or penetrate the coating.

It is preferable for the friction stir processing method to be afriction stir welding method for connecting a first workpiece and asecond workpiece by means of friction stir processing along a weld seam,wherein the first workpiece is provided with a surface coating, and thefriction stir processing method further comprises: engaging the surfacecoating using the first surface region and transferring axial forces bymeans of the second surface region.

A particularly preferred embodiment of the friction stir welding methodis characterized by connecting a first workpiece, which is formed fromgalvanized steel, to a second workpiece, which is formed from a lightmetal, along the weld seam while pressing the first surface region ontothe zinc layer of the first workpiece and pressing the second surfaceregion onto the light metal of the second workpiece.

A further preferred embodiment of the friction stir welding method ischaracterized by using a friction stir processing tool having off-centerpartition line between first surface region and second surface regionand carrying out the friction stir welding method using friction stirprocessing tool, which is to be moved offset in relation to theconnecting line between first workpiece and second workpiece more towardthe second workpiece in the advance direction, wherein the distance ofthe partition line from the center line is selected as a function of thematerials to be connected such that the partition line may be kept inthe vicinity of the weld seam. The partition line is preferably to bekept and the distance X is to be selected precisely at the transitionbetween further surface-coated workpiece surface region and plasticizedweld seam region.

Exemplary embodiments of the invention will be explained in greaterdetail hereafter on the basis of the appended drawings.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

In the figures:

FIG. 1 shows a schematic illustration of a first workpiece having asurface coating and a second workpiece, which are to be connected to oneanother along a connecting line by a friction stir welding method;

FIG. 2 shows a schematic illustration of a friction stir processing toolfor carrying out a friction stir processing method in the form of afriction stir welding method for connecting the workpieces shown in FIG.1;

FIG. 3 shows a top view of a first embodiment of the friction stirprocessing tool shown in a side view in FIG. 2;

FIG. 4 shows a top view of a second embodiment of the friction stirprocessing tool shown in a side view in FIG. 2;

FIG. 5 shows a schematic illustration of a friction stir processing toolfor carrying out the friction stir process using the embodiments of thefriction stir processing tool shown in FIGS. 2 and 3;

FIG. 6 shows a schematic simplified illustration of a friction stirwelding method, which is to be carried out using the friction stirprocessing device according to FIG. 4, for connecting the workpiecesshown in FIG. 1; and

FIG. 7 shows a top view of a third embodiment of the friction stirprocessing tool.

DETAILED DESCRIPTION

FIG. 1 shows a first workpiece 10 and a second workpiece 12, which areto be welded along a connecting region 14 by a friction stir process.The first workpiece 10 is provided in this case with a surface coating16.

In one example, the first workpiece 10 is formed by a galvanized steelplate 18, in which the surface coating 16 is formed by a zinc layer 20.

The second workpiece 12 can be an arbitrary workpiece connectablecorrespondingly to the steel plate 18. For example, the second workpiece12 is formed from a light metal, for example, an aluminum alloy or thelike.

In FIGS. 2, 3, and 4, a partial element of a friction stir processingtool 30 for carrying out the friction stir welding method for connectingthe first workpiece 10 and the second workpiece 12 along the connectingregion 14 is shown.

This partial element of the friction stir processing tool 30 is a body32, on which a shoulder 34 is provided for pressing against a surface 22of the workpieces 10, 12 along the connecting region 14.

The body 32 having the shoulder 34 is, as described in greater detail inGerman patent document DE 10 2005 030 800 B4, assembled together with aseparately provided pin 36 (also called welding pin or probe, see FIG. 5in this regard), which is drivable to rotate, to form the friction stirprocessing tool 30. In this case, the body 32 is rotatable in relationto the pin 36, so that the shoulder 34 can be held stationary and notrotate during the friction stir processing operation—i.e., a stationaryshoulder 34 is used.

In FIGS. 2, 3, and 4, a center plane 42, which includes the center axis40 (coincident with the axis of rotation of the pin) through the body 32is shown, this plane dividing the body 32 into two halves in animaginary manner.

The shoulder 34 has the friction surface 44, which rubs on the surface22 of the workpieces 10, 12.

This friction surface 44 is divided into a first surface region 46 onthe first half and a second surface region 48 on the second half. Stillfurther surface regions can also be provided in embodiments which arenot shown.

In the exemplary embodiments of the friction stir processing tool 30shown in FIG. 3 and FIG. 4, the shoulder 34 is implemented as multipart,so that the surface regions 46, 48 are implemented on different parts ofthe shoulder 34.

A friction reducing unit 50 is provided, by which the frictionresistance of at least one partial region of the friction surface 44 isreduced during rubbing over the surface 22 in relation to a body withoutfriction reducing unit.

The friction reducing unit 50 is implemented for selective reduction ofthe friction resistance on a partial region of the friction surface 44.

In the embodiments shown in FIG. 3 and FIG. 4, the friction reducingunit 50 is assigned to the first surface region 46, so that a frictionresistance between the first surface region 46 and the surface 22 isless than a friction resistance between the second surface region 48 andthe surface 22.

In one embodiment of the friction reducing unit 50, the first surfaceregion 46 is implemented on a support element 52 for this purpose, whichis embodied as softer or more yielding than a supporting surface 54 ofthe second surface region 48.

For example, the support element 52 is formed from plastic or similarmaterials, while the supporting surface 54 is formed from metal materialand/or ceramic material.

These different materials can be formed by the different partialelements of the shoulder 34.

When carrying out the friction stir welding method and connecting thefirst workpiece 10 and the second workpiece 12, the body 32 having theshoulder 34 is aligned in this case so that the first surface region 46engages on the surface coating 16 of the first workpiece, while thesecond surface region 48 engages on the surface of the second workpiece12. In this orientation, the friction stir processing tool 30 is thenmoved during the friction stir welding method along the connectingregion 14, e.g., the connecting line, to thus form a weld seam 46.

In FIG. 3, the size of the first surface region 46 is equal to that ofthe second surface region 48, so that the friction reducing unit 50occupies essentially half of the friction surface 44.

FIG. 4 shows an embodiment in which the first surface region 46 occupiesa smaller part of the friction surface 44 than the second surface region48. The friction reducing unit 50 occupies essentially only the regionbeyond the connecting region 14 to be formed during the weldingoperation here, as will be explained in greater detail hereafter.

In FIGS. 3 and 4, the first surface region 46 and the second surfaceregion 48 are separated by a partition line 57. The surface regions 46and 48 are implemented, for example, in the form of a crescent, sickle,or ring segment. The partition line 57 can lie, as shown in FIG. 3,centrally on the center line of the friction surface 44, i.e., on thecenter plane 42. The partition line 57 can also, however, as shown inFIG. 4, be arranged remotely at a distance X from the center plane 42,preferably extending parallel thereto. For example, an assortment offriction stir processing tools can be provided, in the case of which thedistance X varies. The selection of the friction stir processing tooland thus the selection of the distance X is performed depending on thewelding task.

The friction stir processing tool 30 does not have to be positionedcentrally in the joint during the generation of a mixed connectionbetween two different workpieces. To protect the friction stirprocessing tool against wear, for example, in the case of a connectionof joining partners having significantly different melting or softeningtemperatures, one proceeds such that the tool may penetrate onlyminimally into the respective harder joining partner. In such cases, forexample, in the case of welding of workpieces made of steel or titanium,on the one hand, to a workpiece made of light metal such as aluminumalloy, on the other hand, an application and movement of the frictionstir processing tool is advantageously performed off-center to theconnecting line between the workpieces. The distance X can be selectedcorresponding to the respective advantageous off-center offset, asindicated in FIG. 4.

FIG. 5 shows, to illustrate the performance of the friction stir weldingmethod, a friction stir processing device 58 in the form of a frictionstir welding device 60, which has the friction stir processing tool 30having the body 32 and the pin 36 driven so it is rotatable thereto, andalso a tool guiding unit 62, using which the friction stir processingtool 30 is movable in a first direction R1 and a second direction R2, toguide it along the connecting region 14. Of course, a verticaladjustment can also be provided to move the friction stir processingtool 30 in the third dimension.

The friction stir welding device 60 furthermore has a rotary drive 64for the rotational driving of the pin 36—which can also be referred toas a welding pin or probe—and a pivot unit 66 for aligning the body 32having the shoulder 34 such that the first surface region 46 having thereduced friction is placed on the surface coating 16. The pivot unit 66has a stepping motor 68, which holds the body 32 in the matchingposition via a gear wheel 70 and a gear ring 72. This is all controlledvia a controller 76.

Rotary drive 64, pin 36, and body 32 are held on a housing or a frame74, which is movable by means of the tool guiding unit 62 in therespective desired directions R1 and R2.

Instead of the simple tool guiding unit 62 like a gantry crane shown inFIG. 5, of course, many other tool guiding units 62 can also be used,for example, robot arms or the like.

FIG. 6 shows how the friction stir processing tool 30 is moved along theconnecting region 14 in the advance direction V, wherein theplasticization of the workpiece material and the formation of the weldseam 56 are performed by rotating the pin 36. In this case, the firstsurface region 46 rests on the surface coating 16, while the verticalprocess forces are transferred more via the second surface region 48.

While the friction stir processing tool 30 is moved along the connectingregion, the pivot drive 66 aligns the friction stir processing tool 30so that the center plane 42 extends substantially parallel to theadvance direction V. In this case, it can be preferable, depending onthe type of the materials, for the pin 36 and therefore the center axis40 and therefore in turn the center plane 42 to not be arrangedprecisely centrally to the connecting region 14, but rather offset inthe direction perpendicular to the advance direction toward the centerof the connecting region 14, for example, so that the pin 36 onlyminimally penetrates into one of the workpieces 10, 12 and penetratesmore into the other of the workpieces 10, 12.

For this purpose, a friction stir processing tool 30 according to FIG. 3or FIG. 4 having corresponding matching distance X is selectedbeforehand during the preparation of the welding method, depending onthe advantageous relative location of the center plane 42 of thefriction stir processing tool 30 in relation to the connecting region14—centered or more or less off-center—and subsequently used.

In other cases (not shown in greater detail), it can be that also thesecond workpiece 12 is provided with the surface coating 16, or frictionstir processing methods are to be carried out on a single workpiecehaving surface coating.

For these purposes, the third embodiment of the body 32 shown in FIG. 7can also be used, wherein the friction reducing unit 50 acts on theentire friction surface 44, to reduce its friction resistance duringapplication to the surface coating 16. For example, the frictionreducing unit 50 is provided on the entire circumference of the shoulder34.

In a concrete example of the multipart shoulder of FIG. 3 or FIG. 4, thesupport element 52 is formed from PTFE, while the second element of theshoulder 34, which forms the supporting surface 54, is formed fromsteel.

In the exemplary embodiment of FIG. 7, the friction surface 44 restsentirely on the ring-shaped support element 52 made of PTFE.

Although the friction stir processing tool 30 and the friction stirprocessing device 58 have been described on the basis of the example ofa tool or a device, respectively, for carrying out a friction stirwelding method, the application is not thus restricted to this case.Other friction stir processing methods can also be carried out usingsuch friction stir processing tools.

The advantages of the above-described embodiments will be explained ingreater detail hereafter.

During the application of a friction stir welding process to coatedjoined parts—workpieces 10, 12—damage to the coating occurs due to theshoulder 34 of a heretofore conventional welding tool. Heretofore, theprocedure, that the coating of the joined parts has subsequently beenreapplied locally in the weld seam region, has been used.

In contrast, if a shoulder is used on a body 32, as shown in FIGS. 2, 3,4, and 7, the surface coating 16 may then be protected by a frictionreduction. In particular, friction stir welding of galvanized steel maytherefore be carried out without damaging the zinc layer. A stationaryshoulder 34 is preferably used, which is softer than the zinc layer.

In particular in the case of mixed connections between aluminum andsteel, a further preferred solution is to form one part of the weldingtool shoulder 34 as metallic or ceramic, to be able to transfer asufficient vertical process force, and to form a further part of theshoulder 34 as organic, for example, from PTFE, to generate littlefriction and to protect the surface of one joining partner.

Reworking of the welding for the purpose of renewing a previous coatingis no longer necessary.

In particular, in one embodiment, a body 32 of a friction stirprocessing tool 30 having a multipart shoulder 34 is provided. Thedistribution of the individual parts on the shoulder 34 is selectedaccording to the welding task to be achieved. Depending on welding tasksto be carried out, a central partition line 57 or an off-centerpartition line 57 having advantageous distance X to be determined fromthe center plane 42 can be selected. One criterion for the selection isthat a surface coating 16 is protected and maintained as cleanly aspossible by contact with friction-reducing friction surface, whileimpervious tool surfaces and/or the plasticized region can be contactedby a friction surface optimized for the highest possible pressuretransfer.

The friction stir welding method is applied in particular in theconstruction of aircraft, where workpieces having different materialproperties are to be connected to one another. The requirement alsoexists more and more in automotive engineering of connecting steel partsand aluminum parts to one another. This is now possible more easily thanheretofore using the tool proposed here.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

LIST OF REFERENCE NUMERALS

-   10 first workpiece-   12 second workpiece-   14 connecting region-   16 surface coating-   18 steel plate-   20 zinc layer-   22 surface-   30 friction stir processing tool-   32 body-   34 shoulder-   36 pin-   40 center axis-   42 center plane-   44 friction surface-   46 first surface region-   48 second surface region-   50 friction reducing unit-   52 support element-   54 supporting surface-   56 weld seam-   57 partition line-   58 friction stir processing device-   60 friction stir welding device-   62 tool guiding unit-   64 rotary drive-   66 pivot unit-   68 stepping motor-   70 gear wheel-   72 gear ring-   74 frame-   76 controller-   R1 first direction-   R2 second direction-   V advance direction-   X distance between partition line and center plane

What is claimed is:
 1. A friction stir processing tool, comprising: abody; a shoulder disposed on the body of the friction stir processingtool, wherein the shoulder includes a friction surface configured tocontact at least one workpiece, wherein the workpiece is at leastpartially provided with a surface coating; and a friction reducing unitconfigured to reduce a friction resistance between the surface coatingand the friction surface, wherein the friction surface comprises a firstsurface region and a second surface region, and the friction reducingunit is arranged such that the friction resistance on the first surfaceregion of the friction surface is less than the friction resistance onthe second surface region of the friction surface.
 2. The friction stirprocessing tool of claim 1, wherein the friction reducing unit has atleast one support element configured to support the first region of thefriction surface such that the first region of the friction surface ismore yielding than the surface coating of the at least one workpiece. 3.The friction stir processing tool of claim 2, wherein the at least onesupport element is configured to support the first surface region and issofter or more yielding than a supporting surface of the second surfaceregion.
 4. The friction stir processing tool of claim 1, wherein thefirst surface region is arranged on a region of the body formed from aplastic material and the second surface region is implemented on aregion of the body formed from a metal or ceramic material.
 5. Thefriction stir processing tool of claim 1, wherein the first surfaceregion is formed by a first half of the friction surface, and the secondsurface region is formed by a second half of the friction surface. 6.The friction stir processing tool of claim 1, wherein the first surfaceregion is arranged on a partial segment of the friction surfaceoccupying less than half of the friction surface.
 7. The friction stirprocessing tool of claim 1, wherein a partition line, which separatesthe first surface region from the second surface region, extendsoff-center in relation to the friction surface, so that the secondsurface region extends by a predefined distance beyond a center line ofthe friction surface, which can be arranged in an advance direction forthe friction stir processing tool.
 8. The friction stir processing toolof claim 1, further comprising: a pin, which protrudes from the shoulderand which is rotatable in relation to the shoulder.
 9. The friction stirprocessing tool of claim 8, wherein the shoulder is arranged so that,during a friction stir welding method for connecting a first workpieceand a second workpiece along a weld seam, the shoulder is pressedagainst a surface of the first workpiece or the second workpiece at arotational speed of
 0. 10. A friction stir processing device,comprising: a friction stir processing tool, which comprises a body; ashoulder disposed on the body of the friction stir processing tool,wherein the shoulder includes a friction surface configured to contactat least one workpiece, wherein the workpiece is at least partiallyprovided with a surface coating; a friction reducing unit configured toreduce a friction resistance between the surface coating and thefriction surface, wherein the friction surface comprises a first surfaceregion and a second surface region, and the friction reducing unit isarranged such that the friction resistance on the first surface regionof the friction surface is less than the friction resistance on thesecond surface region of the friction surface; and a tool guiding unitconfigured to guide and move the friction stir processing tool in anadvance direction.